Objective To explore the effects and mechanism in patients with ulcerative colitis (UC) treated by Yin Yang equilibrated preparation. Method 82 patients with UC as treatment group and 35 health parsons as control group underwent the testing. The serum and mucosal of all testing persons were taken pre and post treatment to detect CD+4 CD+25 regulatory T cells(Tregs) by flow cytometry at the end of one month,treating with SASP and Yin Yang equilibrated preparation. Result Compared with normal control group,the proportion of CD+4 CD+25 Tregs was markedly decreased in PB and mucesal of group A and B(P <0.01). But it was significanfly increased in therapeutic groups of SASP and Yin Yang equilibrated preparation, and their CD+4 CD+25 Tregs in PB and mucosal were much more than the control group at the end of one month after treating with SASP and Yin Yang equilibrated preparation(P <0.01 or P< 0.05). Conclusion CD+4 CD+25 Tregs with strong immune suppression play a central role in the initia-tion and development of UC,and the Yin Yang equilibrated preparation might exert its therapeutic effects on UC by the regulation of number and function of CD+4 CD+25 Tregs.